I’d say that not many (any) cloud remotes support ctime/atime so I’m not sold on the idea that they need to be synced. I don’t think you can write atime’s at all to local disk so they can’t be synced to the local backend.

To read them I’d make an Optional interface AccessTime & ChangeTime and only implement that on the local remote…

Other than syncing it what could you do with mtime and atime? You can write ctimes though.

I see the metadata thing a separate discussion probably.

Perhaps to solve this particular issue, a flag like --local-times mtime/ctime/atime might be best so the local backend reads just the time specified as the modtime.